My main prob with bp is based on my own experience - I have differed in weight (im at the high end now) by 60 pounds. There is no difference in my nbp compared to when I was 60 pounds lighter. Actually, my nbp is slightly longer now because of my pe. But it was the same before my pe. It looks smaller now because the rest of me is bigger. But it is the same length with 60 more pounds on me. Any doctor will tell you losing weight does not make your dick bigger, it only makes it appear bigger because you can see more clearly what is going on as you are coming off a flater base.
Now, if I had been measuring bp all along, the extra fat in that area would definitely have affected measurement. So instead of getting the true measurement, you are measuring fat differences in that area - again nothing to do with penis. So, you could gain or lose weight during a course of pe and think your penis lost or gained length when all that really happened was you lost or gained some fat in that area.